Method of passivating an iron disulfide surface via encapsulation in zinc sulfide

ABSTRACT

A method for passivating the surface of crystalline iron disulfide (FeS 2 ) by encapsulating it in crystalline zinc sulfide (ZnS). Also disclosed is the related product comprising FeS 2  encapsulated by ZnS in which the sulfur atoms at the FeS 2  surfaces are passivated. Additionally disclosed is a photovoltaic (PV) device incorporating FeS 2  encapsulated by ZnS.

PRIORITY CLAIM

The present application is a non-provisional application claiming thebenefit of U.S. Provisional Application No. 61/954,703, filed on Mar.18, 2014 by Jesse A. Frantz et al., entitled “Method of Passivating anIron Disulfide Surface via Encapsulation in Zinc Sulfide,” the entirecontents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to passivating the surface of crystallineiron disulfide (FeS₂) by encapsulating it in crystalline zinc sulfide(ZnS). It also relates to FeS₂ encapsulated by ZnS in which the sulfuratoms at the FeS₂ surfaces are passivated. Additionally, this inventionrelates to a photovoltaic (PV) device incorporating FeS₂ encapsulated byZnS.

Description of the Prior Art

Iron disulfide has great promise as an Earth-abundant material for PVapplications. In a recent survey of 23 known semiconductor systems withpotential as PV absorbers, FeS₂ ranked highest in potential annualelectricity production based on known reserves and had the lowestextraction cost. Wadia et al., “Materials availability expands theopportunity for large-scale photovoltaics deployment,” Environ. Sci.Technol., 43, pp. 2072-2077 (2009). Its bandgap is 0.95eV, high enoughto result in a potential solar to electricity conversion efficiencysimilar to that of Si, but unlike Si, it has an exceptionally highabsorption coefficient of α=6×10⁵ cm⁻¹ resulting in a required thicknessof <40 nm for >90% absorption (Ennaoui et al., “Iron disulfide for solarenergy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993))compared to typical thicknesses >100 μm for Si.

Despite these apparent advantages, FeS₂ PV devices have not yet lived upto their potential. Efficiency has been limited to approximately 3% dueto open circuit voltage (V_(OC)) of <200 mV, only ˜20% of the bandgap.Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol.Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993) and Wilcoxon et al.,“Strong quantum confinement effects in semiconductors: FeS₂nanoclusters,” Sol. State Comm., 98, pp. 581-585(1996). These limitshave been shown to be the direct result of surface termination of FeS₂crystals. Bulk FeS₂ crystallizes in the cubic pyrite structure, and itssulfur atoms are paired in an S-S bond (S₂ ²⁻). Crystal surfaces,however, are typically terminated by S monomers (S¹⁻) that may convertto S²⁻through a redox reaction. Bi et al., “Air stable, photosensitive,phase pure iron pyrite nanocrystal thin films for photovoltaicapplication,” Nano Lett., 11, pp. 4953-4957 (2011) and Zhang et al.,“Effect of surface stoichiometry on the band gap of the pyrite FeS₂(100)surface,” Phys. Rev. B, 85, art. 085314 (2012). The resulting surfacestates exhibit a high density of defects within the FeS₂ bandgap andshows properties similar to the iron monosulfide phase, with a bandgapof approximately 0.3 eV. In PV devices, these surface states at films'surfaces and grain boundaries lead to high dark current and low V_(OC−).

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a method of passivating the surface of crystalline irondisulfide (FeS₂) by encapsulating it in crystalline zinc sulfide (ZnS),a product comprising FeS₂ encapsulated by ZnS in which the sulfur atomsat the FeS₂ surfaces are passivated, and a photovoltaic (PV) deviceincorporating FeS₂ encapsulated by ZnS.

To move beyond its current performance bottleneck, FeS₂ requirespassivation of its surface states. The present invention provides amethod of passivating surface defects in FeS₂ by encapsulating it inZnS. A density-functional theory (DFT) study indicates that ZnS cancreate a defect-free interface with FeS₂. Experimental results indicatethat surface defects in polycrystalline FeS₂ films are indeed passivatedby encapsulation in ZnS.

The present invention has many advantages. It results in passivation ofsurface states in crystalline FeS₂, which is a feature that has not beenshown with any other encapsulant. Using the method of the presentinvention, the FeS₂/ZnS interface can be made free of mid-gap statesthat are typically associated with S monomers at the FeS₂ surface. Also,this invention can be incorporated into a rigid or flexible PV device,making it possible to build efficient solar cells from an Earth-abundantmaterial.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a polycrystalline FeS₂ film encapsulated by coating it witha ZnS capping layer.

FIG. 2 shows XPS results comparing the S 2P peak. FIG. 2a is for FeS₂encapsulated by a ZnS capping layer. FIG. 2b is for FeS₂ encapsulated bya ZnO capping layer. FIG. 2c is for FeS₂ encapsulated by a SiO₂ cappinglayer.

FIG. 3 shows the atomic structure of an FeS₂ nanocrystal embedded in aZnS matrix, based on DFT results.

FIG. 4 shows FeS₂ crystallites encapsulated within a ZnS matrix. In FIG.4a , the substrate is a rigid material. In FIG. 4b , the substrate is aflexible material.

FIG. 5 shows FeS₂ crystallites encapsulated within a ZnS matrix beingemployed in a PV device.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, FeS₂ is sputtered at room temperature from a singletarget in a partial pressure (1×10−5 T) of sulfur onto a glasssubstrate. The film was ˜200 nm thick and polycrystalline. It exhibitedthe expected cubic pyrite crystal structure as indicated by X-raydiffractometry. The sample was transferred to an evaporation chamberwithout removal to atmosphere, and a 40 nm thick layer of ZnS wasdeposited by thermal evaporation. A sketch of this sample is shown inFIG. 1. Transferring the sample between deposition chambers under vacuumavoids oxidation and contamination. Alternatively, high substratetemperature deposition of FeS₂ may be carried out by sputtering from amulti-component target to a high temperature substrate (T_(s) =400° C.),and sulfurdizing under flowing H₂S at 500° C., for 5 hours. Allvacuum-deposited fabrication results in high quality films.

Initial X-ray photoelectron spectroscopy (XPS) results for this samplewere obtained and compared to results for bare FeS₂ and films with ZnS,ZnO and SiO₂ encapsulation layers. The encapsulation layers were removedin steps inside an ultra-high vacuum chamber with an ion beam, and XPSscans were carried out after each removal step. The results, shown inFIG. 2, compare the S 2p doublets of 400 nm thick FeS₂ films with 40 nmthick ZnS, ZnO, and SiO₂ capping layers. In each case a combination of S2p doublets associated with both the bulk states and surface defects ispresent. The peak with lowest binding energy (near −161 eV) is the S 2p_(3/2)component of the doublet and is associated with these surfacedefects. For both ZnO and SiO₂, this peak is stronger than the peakassociated with the bulk states, indicating a larger concentration ofsurface defects, presumably S²⁻. For the ZnS-capped sample, however, thedefect peak is smaller relative to the bulk peak, indicating that thesurface defects have been partially passivated. This is the firstdemonstration of passivation of FeS₂ surface defects by a ZnS cappinglayer.

To obtain an atomic scale understanding of the bonding between FeS₂ andZnS, DFT calculations were carried out. The FeS₂ and ZnS have a nearlyperfect lattice match, with lattice spacings of 5.417 Åand 5.411 Å,respectively, and form an epitaxial layer. Because of this the twomaterials can form a nearly defect-free interface. An illustration of anFeS₂nanocrystal encapsulated in ZnS, based on DFT, is shown in FIG. 3.

Several other embodiments of the invention are shown in FIG. 4. In thesecases, FeS₂ crystallites are encapsulated within a ZnS matrix. The FeS₂crystallites may vary in size from 1 nm to 10 μm, and the ZnS separatingFeS₂ crystallites is at least one monolayer thick. FIG. 4a shows anembodiment in which the substrate is a rigid material such as rigidglass or a semiconductor wafer; and FIG. 4b shows an embodiment in whichthe substrate is a flexible material such as polymer, flexible glass, ormetal foil. In the latter case, the FeS₂ and ZnS matrix constitute afilm that may flex along with the substrate.

In another embodiment, the film comprising FeS₂ crystallitesencapsulated within a ZnS matrix is employed as the absorber in a PVdevice. One example of a suitable device architecture is shown in FIG.5. In this example, the device comprises a substrate, a conductivebottom contact, the FeS2 crystallites encapsulated within a ZnS matrix,a transparent p-type layer, a transparent conductive to serve as the topcontact, and a metal grid that aids efficient charge collection. FeS₂ istypically an n-type semiconductor, so in this architecture, thetransparent p-type layer is used in conjunction with the FeS₂/ZnS layerto form a p-n junction. Any other suitable PV device architecture, suchas a Schottky junction device, could be used.

The FeS₂ crystallite size may vary from 1 nm to 10 cm. Individualcrystallites may be in contact, as is the case in polycrystalline bulksamples or thin films, or crystallites may be separated with eachentirely encapsulated in ZnS. The FeS₂ may be a natural or syntheticbulk sample.

The FeS₂ may be film deposited by any suitable deposition technique.This technique may be any physical vapor, chemical vapor deposition,atomic layer deposition, or other suitable deposition process.

The ZnS may be a film deposited by any suitable deposition technique.This technique may be any physical vapor, chemical vapor deposition,atomic layer deposition, or other suitable deposition process. The Scontent in FeS₂ could vary by up to ±20% from stoichiometry.

The Fe in FeS₂ could be partially substituted by Si with a ratio of upto 50%, i.e. Fe_(1−x)Si_(x)S₂ where x<0.5. The Zn in ZnS could bepartially substituted by another metal including Ni, Mn, Cu, Ag, or Pbwith a ratio of up to 50%. The S in ZnS could be partially substitutedby Se or O with a ratio of up to 50%.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for passivating a surface ofcrystalline iron disulfide, comprising: sputtering iron disulfide toform a layer of crystalline iron disulfide on a substrate, wherein thelayer has a surface comprising crystal surfaces; and depositing acapping layer of epitaxial zinc sulfide onto the surface of thecrystalline iron disulfide layer under vacuum, wherein the crystalsurfaces of the crystalline iron disulfide are encapsulated by theepitaxial zinc sulfide capping layer; wherein the epitaxial zinc sulfidecapping layer passivates sulfur atoms present on the crystal surfaces onthe surface of the crystalline iron disulfide layer, thereby reducingsurface defects as compared to a crystalline iron disulfide layer notcapped with a zinc sulfide capping layer.
 2. The method of claim 1,wherein the substrate is a rigid material.
 3. The method of claim 1,wherein the substrate is a flexible material.
 4. The method of claim 1,wherein the crystalline iron disulfide comprises crystallites ranging insize from 1 nm to 10 μm.
 5. The method of claim 1, wherein the surfacedefects in the crystalline iron disulfide layer are assessed bycomparing an X-ray photoelectron spectroscopy scan of S 2p doubletsassociated with surface defects with an X-ray photoelectric spectroscopyscan of S 2p doublets associated with the bulk state.
 6. The method ofclaim 1, wherein the crystal surfaces on the surface of the layer ofcrystalline iron disulfide and the capping layer of epitaxial zincsulfide form a lattice match.
 7. The method of claim 1, wherein thecapping layer of epitaxial zinc sulfide has a lattice constant of about5.411 Å.
 8. The method of claim 1, wherein the crystal surfaces on thesurface of layer of crystalline iron disulfide have a lattice constantof about 5.417 Å.
 9. The method of claim 1, wherein the capping layer ofepitaxial zinc sulfide is deposited by physical vapor deposition. 10.The method of claim 1, wherein the capping layer of epitaxial zincsulfide is deposited by chemical vapor deposition.
 11. The method ofclaim 10, wherein the chemical vapor deposition is atomic layerdeposition.
 12. The method of claim 1, wherein the layer of crystallineiron disulfide is deposited by physical vapor deposition.
 13. The methodof claim 1, wherein the layer of crystalline iron disulfide is isdeposited by chemical vapor deposition.
 14. The method of claim 13,wherein the chemical vapor deposition is atomic layer deposition.